At this point, and knowing that AC/SG will be shared between PSO2 and PSO2: NG, it is best to save all of our SG for New Genesis as it is clearly the future of Phantasy Star Online and it will probably have as many uses as it does in PSO2.
Especially considering that SG is much harder to come by IN PSO2NA compared to PSO2JP and that the SG scratches are basically a bottomless pit due to being untradable and having so much filler that reduces the % of getting what you want.
This is especially valid for free2play players that do not expect to or are able to spend money on the game and thus have very limited supplies of SG that may not even be enough to get a full costume set already as is.
After all, we do not even know if our hard earned SG costumes and fashion will carry over, so it may be a waste even if you get them, especially depending on WHEN in 2021 the game releases.
I am sure that starting New Genesis with a generous amount of supplies and SG will be beneficial.